drm/i915/gsc: Fix error code in intel_gsc_uc_heci_cmd_submit_nonpriv()

This should return negative -EAGAIN instead of positive EAGAIN.

Fixes: e5e1e6d2 ("drm/i915/pxp: Add MTL helpers to submit Heci-Cmd-Packet to GSC")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ZH7sr+Vs4zOQoouU@moroto